Photo cross-linkable liquid crystals provided with a suitable amount of a photoinitiator can be orientated by an orienting layer on a substrate such as glass or silicon or in an electric or magnetic field and can then be cross-linked by irradiation with light of a suitable wavelength. The thus-produced orientation remains even at high temperatures. This also applies when the orientating layer consists of photo-orientable polymer network layers (PPN) which induce a high resolution orientation pattern on liquid crystal layers by selective radiation with polarized UV light. Such hybrid layers make it possible to transfer the optically registered high resolution orientation structure of a thin polymer layer to a double refractive densely cross-linked polymer layer of any thickness. Mixtures consisting of photo cross-linkable liquid crystals or photo cross-linkable liquid crystals and low molecular liquid crystals and/or optically active (chiral) additives are used depending on the application. Thus, optical building elements having long-term stability, such as, for example, optical retarders, wave guides, optical grids and filters, integrated color filters, piezo-electric optical building elements and those having non-linear optical (NLO) properties, and the like can be manufactured. Such optical building elements find use, for example, in projection systems.
Further properties, such as, for example, the birefringence, the refractive index, the transparency, etc. must fulfil different requirements depending on the field of application. For example, materials for optical retarders should have a high birefringence in order that the layer thickness of the integrated optical building element can be kept to a minimum.
Besides the general interest in photo cross-linkable liquid crystals for optical building elements, such liquid crystalline materials are suitable for cladding glass fibres for optical data transmission. The use of such materials increases the elastic modulus in the longitudinal axis of the fibre, lessens the thermal expansion coefficient and reduces microdistortion losses. This leads to an increased mechanical stability.
The photo cross-linkable liquid crystals must have a good chemical and thermal stability, a good solubility in usual solvents and a good stability towards electric fields and electromagnetic radiation. Furthermore, they should have a suitable mesophase from about 25.degree. C. to about 80.degree. C., when possible from about 25.degree. C. to about 100.degree. C., for example a broad smectic or nematic mesophase and, respectively, a chiral smectic and cholesteric mesophase, for the applications mentioned above.
Since liquid crystals are usually used as mixtures of several components, it is important that the components have a good miscibility with one another.